Osmotic potential (it.) of aqueous solutions of polyethylene glycol 6000 (PEG-6000) was curvilinearly related to concentration. At given concentrations, AI. increased linearly with temperature. The effects of concentration and temperature on ,1. of PEG-6000 solutions differ from those for most salts and sugars and apparently are related to structural changes in the PEG polymer. Measurements of t'. with thermocouple psychrometers are more negative than those with a vapor pressure osmometer, with the psychrometer probably giving the more nearly correct i1, for bulk solutions. An empirical equation permits calculation of A1, from known concentrations of PEG-6000 over a temperature range of 15 to 35 C. Viscometery and gravimetric analysis are convenient methods by which the concentrations of PEG-6000 solutions may be measured.The use of polyethylene glycol to adjust osmotic potential (s,) requires accurate knowledge of the effect of PEG2 on if. The calculation of i, from freezing point depression is unsatisfactory (9). Thermocouple psychrometry (9,11,13,(19)(20)(21) and vapor pressure osmometry (7,12,17) have been employed; however concentrations used to achieve f, values frequently have not been given (7,(11)(12)(13)(14)17) or were reported graphically (4,9,(19)(20)(21)

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Tree age and size structures were compared within and among topographic categories in portions of a 35-km 2 unlogged landscape and a comparable adjacent logged landscape. Tree density was generally higher in the logged landscape. One fifth of plots in the unlogged landscape had trees older than 400 years, but no trees older than 400 years remained in the logged landscape plots. Ten recruitment pulses were identified for the unlogged study area, accounting for 49% of all trees measured during 26% of the 421-year survival record. Recruitment pulses in the logged area accounted for fewer trees during a larger amount of time. Most recruitment periods in the unlogged landscape coincided with known past major fires. The mixed-severity historical fire regime created openings that persisted for as long as 148 years. The following components exist in the unlogged landscape: (i) forest patches having a distinct age cap reflecting regeneration following an earlier stand-replacing fire, (ii) uneven-aged forest patches having no evidence of an age cap, (ii) openings created by fire, and (iv) riparian areas. Results suggest that the logged landscape is poised to regain an old-growth age distribution, and tree removal in the logged landscape could restore the size distribution found in the unlogged landscape. However, the unlogged landscape has openings not found in the logged landscape that should be considered in restoration efforts at a landscape scale. Résumé : Les structures de dimension et d'âge des arbres ont été comparées à l'intérieur et entre des unités topographiques dans des portions d'un paysage de 35 km 2 où il n'y a pas eu de coupe et d'un paysage adjacent et comparable où il y a eu des coupes. La densité des arbres était généralement plus élevée dans le paysage avec des coupes. Dans le paysage sans coupe, une parcelle sur cinq contenait des arbres plus vieux que 400 ans alors qu'il ne restait aucun arbre de 400 ans dans les parcelles du paysage avec des coupes. Dix vagues de recrutement ont été identifiées dans l'aire d'étude non coupée représentant 49% de tous les arbres mesurés pour 26% des 421 années de données de survie. Les vagues de recrutement dans la zone coupée représentaient moins d'arbres pendant une période plus longue. La plupart des périodes de recrutement dans le paysage sans coupe coïncidaient avec d'importants feux survenus dans le passé et dont on connaît l'existence. Le régime passé des feux dont la sévérité était variable a créé des ouvertures qui ont persisté pour aussi longtemps que 148 ans. On trouve les composantes suivantes dans le paysage sans coupe : (i) des îlots de forêt dont la limite d'âge indique clairement qu'une période de régénération survenue suite à un feu a entraîné le remplacement du peuplement, (ii) des îlots de forêt inéquienne où la limite d'âge n'est pas évidente, (iii) des ouvertures créées par le feu et (iv) des zones ripariennes. Les résultats suggèrent que le paysage avec des coupes est en voie de retrouver une distribution d'âges typique d'une vieille forêt et que ...

A water flux model, which assumes that the dynamic functioning of the soil‐plant‐atmosphere continuum may be described by a series of steady states, was examined as a means for interpreting leaf water potential measurements in ‘Valencia’ orange trees (Citrus sinensis (L.) Osbeck). According to the model, leaf water potential should be related to transpirational flux, which in this experiment was estimated by the ratio of vapor pressure deficit of the atmosphere to leaf diffusion resistance (VPD/rleaf). Leaf water potentials decreased in a specific relationship with increasing values of VPD/rleaf provided that soil water was adequate and soil temperature was not too low, but regardless of season of the year or climatic or edaphic differences among 3 field locations. When soil water tensions exceeded 0.3 bar or when soil temperatures were lower than 15°C, deviations from the model occurred in the form of more negative leaf water potentials than predicted by VPD/rleaf. The model predicts from simple measurements made on intact plants that these differences were due to the modification of flow resistances by cool temperatures and the modification of both resistances and the potential of water at the source in the case of soil water depletion. The model may be a useful tool for interpreting plant water potential data under contrasting environmental conditions.

Burns, chartered this team to recommend an ecological basis for ecosystem management. This report is not intended to provide details on all aspects of ecosystem management; it simply provides information and makes recommendations for an ecological basis for ecosystem management. The report is not a decision document. It does not allocate resources on public lands nor does it make recommendations to that effect.The report of this Study Team may be relied upon as input in processes initiated under the National Environmental Policy Act (NEPA), National Forest Management Act (NFMA), Endangered Species Act (ESA), Administrative Procedures Act (APA), and other applicable laws. The information contained in this report is general in nature, rather than site specific. Implementation of ecosystem management and allocation of resources on Forest Service administered lands is the responsibility of the National Forest System in partnership with Forest Service Research and State and Private Forestry. Implementation is done through Forest and project plans that are subject to the NEPA process of disclosing the effects of proposed actions and affording the opportunity for public comment. The Southwestern Region follows a planning process for projects called Integrated Resource Management (IRM).The opinions expressed by the authors do not necessarily represent the policy or position of the U.S. Department of Agriculture, the Forest Service, The Nature Conservancy, or the Arizona Game and Fish Department.The Study Team acknowledges the valuable input of more than 50 individuals from various agencies, universities, professional organizations, and other groups who provided thoughtful comments of an earlier draft of this document. Some of their comments are included in Appendix 3. (Upper left) Road expansion to accommodate increased traffic in forested areas. Photo from Bev Driver. (Upper right) Cypripedium calceolus, a rare species found on the Santa Fe National Forest, New Mexico. Fine filter analyses help protect uncommon species. Photo by Reggie Fletcher. (Lower) Small openings in a ponderosa pine forest created by hotspots in a low-intensity fire. Photo by Ron Moody.

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The efficiency with which plants transport water is related to the water potential differences required to drive water fluxes from the soil to the leaf. A comparative study of The extent to which climatic, plant, and edaphic factors influence plant-water balance and plant-water use is both complex and of considerable adaptive significance. Analyses of water movement in the soil-plant-atmosphere continuum and of the development of plant-water stress are facilitated by interpreting plant responses to variations in environment using conceptual and mathematical models. Elfving et al. (4) described leaf water potential (ileaf) as being dependent upon three elements: soil water potential (4'soi) flux of water through the system, and resistance to flow between the soil and the leaf (r,0ji to leaf).4.teaf = 'soil -(flux)(r,oii to leaf) (1) When the soil water supply is optimal, /'soil is approximately zero and the resistance for water flow from soil to root is negligi- (17), and Barrs (1) observed both types of response with different species. These differences may be due to differences in methodology; but it is also possible that important differences exist between species in their ability to transport water. To test these possibilities we examined relationships between 4'leaf and transpirational flux for woody and herbaceous species by varying the evaporative demand with optimal soil water supply in controlled environments. Differences in water transport efficiency may be also coupled with differences in stomatal regulation of water loss from plants. Some authors (7,10,13) have presented evidence for stomatal responses to humidity gradients between leaf and air that are independent of average leaf water status, while others (11) have concluded that stomata are relatively unaffected by changes in external humidity. If stomata do close when the humidity gradient is increased, leaf resistance would also increase and transpirational flux would not respond linearly to changes in the humidity gradient (15). The influence of the humidity gradient on leaf resistance was investigated with two herbaceous species, since the controversy concerning stomatal response to humidity may also be due to differential behavior between species. MATERIALS AND METHODS Orange

The water relations of pepper plants (Capsicum frutescens L) under conditions conducive to guttation were studied to evaluate the control of plant water stress with polyethylene glycols. The addition of polyethylene glycol 6000 to the nutrient solution resulted in water relations similar to those ex-pected in soil at the same water potentials. Specifically, xylem pressure potential in the root and leaf became more negative during a 24-hour treatment period, while osmotic potential of the root xylem sap remained constant. The decrease in pressure potential was closely correlated with the decrease in osmotic potential of the nutrient solution. In contrast, the addition of polyethylene glycol 400 to the nutrient medium resulted in a reduction of osmotic potential in the root xylem sap; this osmotic adjustment in the xylem was large enough to establish an osmotic gradient for entry of water and cause guttation at a nutrient solution osmotic potential of -4.8 bars. Pressure potential in the root and leaf xylem became negative only at nutrient solution osmotic potentials lower than -4.8 bars. About half of the xylem osmotic adjustment in the presence of polyethylene glycol 400 was caused by increased accumulation of K+, Na*, Ca"+, and Mg+ in the root xylem. These studies indicate that larger polyethylene glycol molecules such as polyethylene glycol 6000 are more useful for simulating soil water stress than smaller molecules such as polyethylene glycol 400.Polyethylene glycol compounds of various molecular weights are used extensively for the experimental control of water stress in plants growing in nutrient solutions. It is frequently assumed that plant water relations are similar whether the plants are growing in soil or in a PEG' solution having an equal water potential. Barrs (1) observed, however, that guttation occurred when pepper plants in a humid chamber were placed in a PEG solution having an osmotic potential of -6 bars. He also found an increase in leaf water potential to values less negative than that of the solution. He attributed these observations to an unusually high root pressure.Preliminary experiments with PEG-1000 confirmed Barrs' observations and led to a consideration of the water and salt relations associated with guttation by plants in solutions having reduced osmotic potentials. For guttation to occur, the osmotic potential of the root xylem must be more negative than that of the nutrient medium, establishing a gradient for osmotic entry of water into the root. Therefore, in Barrs' experiment, the 'Abbreviation: PEG: Polyethylene glycol. osmotic potential in the root xylem must have been at least as low as -6 bars. An osmotic potential this low is not encountered under nonsaline natural conditions, and it would appear that a large osmotic adjustment in the xylem occurred in the presence of PEG.In the experiments reported here, the water relations of pepper plants during a 24-hr exposure to PEG are examined. One objective is to determine whether PEGs of different molecular weights elici...